1. Field of the Invention
The present invention relates to an exposure device for irradiating a surface to be scanned with a plurality of light beams, and an image forming apparatus, and more particularly to an exposure device that performs light amount control on a plurality of light beams before scanning an image bearing member, such as a photosensitive member, as a surface to be scanned, and performs multiple exposure on the image bearing member with the light beams subjected to the light amount control to thereby form pixels, and an image forming apparatus equipped with the exposure device.
2. Description of the Related Art
There has generally been known an image forming apparatus, such as a copying machine or a printer, which performs image formation using so-called electrophotographic process. In recent years, it has been demanded that an image forming apparatus of this type forms high-quality images on a plurality of types of recording sheets (transfer sheets) at a high speed and with high accuracy.
An image forming apparatus using the electrophotographic process is provided with an optical scanning device (also referred to as “the exposure device”), and exposes a photosensitive member to a light beam emitted from the optical scanning device to thereby form an electrostatic latent image on the photosensitive member. In performing exposure of the photosensitive member, a light beam (also referred to as “a laser beam” or “laser light”) is generated by a beam generator, such as a semiconductor laser, provided in the optical scanning device. The laser beam is deflected by a rotary polygon mirror (hereinafter simply referred to as “the polygon mirror”) driven by a drive motor (hereinafter referred to as “the scanner motor”) for rotation. The deflected laser beam is guided onto the photosensitive member, and the photosensitive member is exposed to the light beam, whereby an electrostatic latent image is formed on the photosensitive member.
In some image forming apparatuses of this type, in order to cope with an increase in printing speed and an increase in resolution, an increased number of beam generators are provided, thereby causing a photosensitive member to be simultaneously exposed to a plurality of light beams. In the case of exposure of a photosensitive member performed by an image forming apparatus using a plurality of light beams, an area exposed by a single exposure operation is larger than in the case of exposure of the same performed by an image forming apparatus using a single light beam. Therefore, the image forming apparatus which exposes the photosensitive member using a plurality of light beams can achieve a faster image forming speed than the mage forming apparatus which exposes the photosensitive member using a single light beam. However, when surface tilt of a polygon mirror occurs, a space interval (pitch) between adjacent line images (scanning lines) each formed using a plurality of light beams during a single scanning operation associated with one face of the polygon mirror differs from a space interval (pitch) between adjacent line images (scanning lines) each formed using the plurality of light beams during a single scanning operation associated with another face of the polygon mirror. Further, non-uniformity of space intervals between the beam generators causes non-uniformity of the space intervals between the line images on the photosensitive member. The non-uniformity of the space intervals becomes visually conspicuous depending on the number of light beams and the resolution of an image forming apparatus, which causes degradation of image quality.
For example, as a method of reducing the above-mentioned non-uniform density, there has been known a method in which an identical spot on a photosensitive member is exposed to a plurality of light beams deflected by respective different reflective surfaces of a polygon mirror (see Japanese Patent Laid-Open Publication No. 2004-109680). The method in which an electrostatic latent image is formed by exposing a once-exposed spot again is called “multiple exposure”. By forming an image by the multiple exposure, a periodic positional displacement caused by a surface tilt of the polygon mirror, different light beam pitches or the like can be made inconspicuous.
However, the apparatus disclosed in Japanese Patent Laid-Open Publication No. 2004-109680 suffers from the following problems: Light amount control is performed so as to hold the light amount of a light beam at a predetermined light amount. The light amount control is performed as follows: First, a light beam emitted from each beam generator is detected by a photodiode (PD), and the light amount of the light beam is detected from a result of the detection. Then, a comparison is performed between the detected light amount and a target light amount, and the value of a drive current to be supplied to the beam generator is controlled such that the light amount of the light beam becomes equal to the target light amount. This method is generally referred to as “APC (automatic power control”. Note that APC is performed by detecting light beams from the respective beam generators using the photodiode in a non-image area during image formation. In an image area immediately after a non-image area within one scanning cycle, a drive current controlled based on a detection result from the photodiode is supplied to each beam generator, whereby a controlled light beam is emitted from the beam generator. Further, for light beams to be emitted from the respective beam generators, APC is performed using the photodiode common to the beam generators, and hence the beam generators emit the respective light beams in timings different from each other during APC in the non-image area.
The image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2004-109680 is configured to form each dot by a plurality of exposure operations, and hence it is required to increase the number of light beams in comparison with a case where each dot is formed by a single exposure operation. When it is required to perform APC in each scanning cycle for each of the light beams emitted from the respective beam generators, time required for execution of APC increases, making it difficult to perform APC for all the light beams in a non-image area between scanning lines. If APC is not performed for some beam generators, variation in light amount is liable to occur between the scanning lines, causing non-uniform density. Further, it is required to provide each of the beam generators with a control circuit for performing APC, which causes an increase in circuit size.